On some salient unresolved issues in severe accidents for advanced light water reactors

Abstract

Some of the outstanding issues of the severe accident are evaluated from the synthesis of the results from the research programs of the simulation of naturally arrested thermal attack in the vessel (SONATA), test for real corium interaction with water (TROI), and the test for the performance of a quenching screen for a hydrogen burn. The strategy of the in vessel retention (IVR) by an external reactor vessel cooling (ERVC) is evaluated for the Korea Standard Nuclear Power Plant (KSNP) and Advanced Power Reactor-1400 (APR-1400). The favorable and adverse effects of the proposed strategy and the uncertainties of the relevant phenomena including the gap cooling and heat removal capability of the EVRC are discussed. It is shown that the thermal margin for the failure of the reactor vessel is challenged for these reactors. The observation of the energetic spontaneous steam explosions in the TROI suggests that proper measures should be taken to avoid and/or lessen the risk of an in-vessel or ex-vessel steam explosion, when we employ an ex-vessel cooling strategy and/or an in vessel injection strategy. To address the issue that multiple burns induced by the hydrogen igniter can pose a threat to the safety equipment, the use of a quenching screen is suggested. The results of the laboratory scale experiments for the performance of the quenching screen indicated the feasibility of using a quenching screen to protect the essential equipment by stopping the flame propagation.